David Kay, the man who led the CIA's postwar effort to find weapons of mass destruction in Iraq, has called on the Bush administration to "come clean with the American people" and admit it was wrong about the existence of the weapons.

In an interview with the Guardian, Mr Kay said the administration's reluctance to make that admission was delaying essential reforms of US intelligence agencies, and further undermining its credibility at home and abroad.

He welcomed the creation of a bipartisan commission to investigate prewar intelligence on Iraq, and said the wide-ranging US investigation was much more likely to get to the truth than the Butler inquiry in Britain. That, he noted, had "so many limitations it's going to be almost impossible" to come to meaningful conclusions.

Mr Kay, 63, a former nuclear weapons inspector, provoked uproar at the end of January when he told the Senate that "we were almost all wrong" about Iraq's weapons of mass destruction (WMD).

He also resigned from the Iraq Survey Group (ISG), which he was appointed by the CIA to lead in the hunt for weapons stockpiles, saying its resources had been diverted in the fight against Iraqi insurgents.

"I was more worried that we were still sending teams out to search for things that we were increasingly convinced were not there," Mr Kay said.

His call for a frank admission is an embarrassment for the White House at the start of an election year. The defence secretary, Donald Rumsfeld, has dismissed Mr Kay's assertion that there were no WMD at the start of the Iraq war as a "theory" that was "possible, but not likely".

In his state of the union speech in January, George Bush did not refer to his prewar claims that Iraq was an "immediate threat" but instead said the ISG had found "weapons of mass destruction-related programme activities".

Mr Kay, who was formerly a UN weapons inspector, called for the president to go further. "It's about confronting and coming clean with the American people. He should say we were mistaken and I am determined to find out why," he said.

A White House official said it was too early to draw conclusions: "The ISG is still working, and the commission on this has not even started."

However, Mr Kay said that continued evasion would create public cynicism about the administration's motives, which he believes reflected a genuine fear of WMD falling into the hands of terrorists. He also said that if the administration did not confront the Iraq intelligence fiasco head-on it would undermine its credibility with its allies in future crises "for a generation".

Mr Kay said that he had become convinced there were no WMD to be found several months ago, before presenting an interim report to Congress last October saying no stockpiles had been found, but he said the CIA and the Blair government were nervous about the impact of his conclusions.

"I think the greatest concern about the report was in London rather than in Washington. It was a different political issue in London than it was here," he said, referring to the storm around the death of his former UN colleague David Kelly.

Mr Kay said he had been expecting Dr Kelly's arrival in Iraq to help the search for biological weapons programmes, and had spoken to him shortly before his death. "He never had any doubts about Iraq's programmes," Mr Kay said.